The invention relates to an optical network and an optical add/drop apparatus used in the optical network.
At the present, in an optical network (which is also called a photonic network), there increases importance of a function “optical add/drop (OADM: Optical Add/Drop Multiplexer)” of dropping and adding a flux of signal light having a specified wavelength from signal light subjected to wavelength division multiplexing (WDM) in an optical node (refer to, e.g., Patent documents 1 and 2).
A conventional typical network is configured by a high-speed backbone network and lower-speed sub-networks connected to the backbone network. In this type of network architecture, respective nodes connected directly to the backbone network and the individual nodes connected to the sub-networks were incapable of performing communications without any restrictions. For example, each node connected directly to the backbone network is at first required to be connected to the sub-network via a relay device located at a terminal station on the backbone network.
With advancement of the optical network, however, there arises a demand for connecting a multiplicity of nodes on the network logically almost at the same time. For example, in a communication system for connecting comparatively proximal nodes between metropolises as called a metropolitan area access network, there is a demand for enabling the communications to be performed without any restrictions by linking the respective nodes on the network logically in a mesh-like configuration. This type of network enabling the nodes to communicate with each other substantially simultaneously is termed a full-mesh network. A low-cost and small-scale full-mesh optical network capable of communicating between the nodes without the restrictions has hitherto been demanded in an office-to-office network, grid computing and so on.
Each of the nodes on the this type of optical network was, however, required to be made capable of transmitting and receiving fluxes of light having different wavelengths corresponding to other respective nodes in order to freely communicate with other nodes on the optical network. Therefore, a node architecture became complicated, and devices that structure the nodes came to a large scale and were expensive.
FIG. 1 shows an example of a network physically taking a so-called full-mesh topology enabling the plurality of nodes to be connected simultaneously. As illustrated in FIG. 1, when configuring the full-mesh optical network, a multiplicity of optical fibers was physically needed. Hence, this network was hard to be built up in the metropolitan area having a small amount of allowable space.
FIG. 2 shows an example of the full-mesh network, which, though physically as a ring network, enables the plurality of nodes to be logically connected simultaneously. In this case, it is required that each node be capable of transmitting and receiving the multiplicity of wavelengths corresponding to the number of nodes on the network. Therefore, each node needs a multiplicity of optical function components such as array waveguide type diffraction grating, a 2×2 optical switch, an optical filter, an optical amplifier and so forth. In this case, each node has a necessity of setting the respective optical function components in linkage, which involves complicated control. Accordingly, there is a case where each node might be upsized as large as a device in a backbone system. The full-mesh network was therefore difficult to be actualized at a low cost. Further, it was also difficult to introduce this network into the metropolitan area having the small amount of allowable space. Moreover, it was also considered that there was no necessity of taking the trouble to introduce the large-scale and high-cost full-mesh network.
Further, in the communications of nowadays, one-to-many communications, which are so-called multicast communications, are requested in addition to one-to-one communications between the respective nodes. Given in the optical networks in FIGS. 3 and 4 are examples of multicasting in which one single node transmits the same information to other plural nodes. In the example in FIG. 3, the respective nodes are allocated with receipt wavelengths and transmission wavelengths corresponding to other transmission partner nodes.
For instance, a node #1 is allocated with transmission wavelengths λ1, λ2 and λ3 to nodes #2, #3 and #4. Further, the node #1 is allocated with receipt wavelengths λ4, λ7 and λ10 from the nodes #2, #3 and #4.
In this type of network architecture, when the node #1 tries to distribute, e.g., the same video information to the nodes #2, #3 and #4 by multicasting, it follows that input ports of the respective nodes corresponding to λ1, λ2 and λ3 defined as optical signals transmitted from the node #1 are occupied. Accordingly, in this case, output ports of the node #1 are all occupied by the multicasting, and it is impossible to perform one-to-one transmission, which is so-called unicast, from the node #1 to other node.
Moreover, the node in the conventional optical network involves using a fixed wavelength drop filter for selecting (which may also be called demultiplexing or dropping) the light having a specified wavelength from wavelength division multiplexed light, and a fixed wavelength add filter for adding (which may also be called inserting or adding) the light having the specified wavelength to the wavelength division multiplexed light. The conventional drop filter had, however, a case of being unable to completely removing the light having a drop target wavelength from the wavelength division multiplexed light. Accordingly, as in the network shown in FIG. 4, there was a case in which it is impossible to use, so to speak, reuse the same wavelength as the received wavelength by way of a wavelength for transmission.
In the example in FIG. 4, the light having the wavelength λ1 is used for the transmission to the node #2 from the node #1, and the wavelength λ4 is employed for the transmission to the node #1 from the node #2. In this type of system, the two wavelengths are required between the two nodes, and there were needed the different wavelengths of which the number is twice the number of combinations of the nodes performing the communications across the network. Hence, this system required extremely a high-band WDM amplifier for amplifying the wavelength division multiplexed light traveling across the optical network, resulting in a high cost for building up the network.                [Patent document 1] Japanese Patent Application Laid-Open Publication No. 11-218790        [Patent document 2] Japanese Patent Application Laid-Open Publication No. 2002-214473        